Art. XLVI.—On the Habit of the Rata (Metrosideros robusta).
[Read before the Auckland Institute, 6th November, 1871.]
The occurrence of several climbing species of Metrosideros in New Zealand, coupled perhaps with the native application of the name “Rata” to the majority of species both scandent and erect, has led to a singular error in connection with the form now under consideration, affording a marked instance of the readiness with which erroneous statements relative to natural phenomema are accepted and repeated, although the exercise of a small amount of observation would suffice to detect the fallacy.
Few persons can have travelled amongst settlers in a forest district in the north without having their attention attracted by distorted giant Ratas, and hearing the commonly received opinion that these immense trees were originally weak climbing plants, the stems of which increased in bulk until they killed the fostering tree which had supported them, and ultimately united to form a solid trunk, perhaps some sixty or seventy feet in length, and with the branches perchance attaining a total height of 100 feet. The frequent repetition of these statements has led to the error being reproduced by many superficial writers on New Zealand, although in the original “Flora Novæ Zelandiæ,” published twenty years ago, the plant is correctly described as never climbing. I copy, almost at random, the following extract respecting
the Rata from Wakefield's “Handbook for New Zealand”:—“Rata (Metrosideros robusta). There are several varieties of this tree—one grows at first as a parasite, creeping in numerous stem-like ropes up the trunks of the other forest trees, gradually enclosing them till they perish, and then uniting to form, a noble tree, taller than that which it has destroyed, with an enormous trunk, but hollow within.”
It is, however, noteworthy that this opinion is not expressed by Dr. Hochstetter and the writers of other standard works on New Zealand, who simply speak of the Rata as a large tree with showy blossoms.
The general resemblance which the foliage and inflorescence of one of the scandent species exhibits to our plant has doubtless contributed to the perpetuity of the mistake. M. florida, which is also called Rata, is a climber in all stages of its existence, but may readily be distinguished by its larger leaves and flowers, its weak stems, and above all by the capsule being included within the calyx tube. More than half the capsule of M. robusta is not included in the calyx tube.
There can be no question that M. robusta is often found destroying trees by which it is supported, and these instances are adduced by the bushman as decisive proof of the climbing habit of the plant, and he attempts to confirm his view by calling the species just mentioned (M. florida) the young state of the destroyer—totally ignorant of the fact that he is confusing two widely separate plants. In reality, however, our plant is exactly the reverse of a climber—the so-called trunks or stems being truly aerial roots, sent down from an epiphytic plant in search of nourishment! The seeds of M. robusta are conveyed by birds, or blown by the wind, amongst the epiphytic masses of Asteliads, Lycopods, and Ferns, so abundant in the trees of the northern forests. In this situation the plant takes root and forms a small bush, for a time obtaining sufficient nourishment from the decaying vegetation in which it is growing, until the limited supply proving insufficient for the increasing demand, its roots stretch boldly down the trunk of the supporting tree in search of that full supply which can only be obtained from the earth. Sometimes only a single root is given off, at others one main root with one or two weaker roots are to be seen, and again several roots of about equal dimensions are to be found, but in nearly all cases the different roots or stems are bound together by smaller roots, which are given off at right angles to the trunk of the supporting tree, and become united with the adjacent main roots by inosculation; not unfrequently masses of fibrous roots are developed, which perish with the increase of the main root, after serving their purpose of deriving temporary nourishment from the atmosphere. In course of time the various stems become inosculated, to a greater or lesser extent, along their course, and the supporting tree is literally strangled by their iron embrace. Notwith-
standing the common belief that the stems ultimately become homogeneous, I have never met with an instance where they have united into a solid trunk; it is certainly true that straight stems of great bulk, sometimes twelve feet in diameter, are to be seen; but this is only the case when a single root stem has been formed, or when the specimen is entirely of terrestrial growth. This may be verified by examining the position of the pith. It is, however, to be noted that when several stems are given off, the pith in each will be found much closer to the side on which the root has been in contact with the supporting tree; this, however, arises chiefly from the unequal pressure to which the root has been subjected during growth. The roots or stems may be met with of all heights up to seventy feet, and from one to twelve feet in diameter.
That the habit of the plant is erect, and not scandent, is demonstrated by the young plants in cultivation in our gardens, and this leads me to mention another peculiarity of this species.* The young cultivated plants are always rigid, erect, and bushy, exactly resembling epiphytic specimens of similar size, or specimens growing on rocks. There is no tendency to a scandent habit, and not until the young plant attains a considerable size does it afford any decided indication of a true arboreal stem. It usually produces a few much-branched stems. This has led to the belief that the plant is naturally a shrub, and only becomes a tree when placed in a position to develop aerial roots. But the opinion cannot be maintained in presence of the occurrence of large terrestrial specimens in many localities. I am fully prepared to admit their rarity when compared with the abundance of specimens of epiphytic origin, still the fact remains that in some localities they are frequent enough to attract the special attention of the bushman, who calls this form the “inland pohutukawa,” a designation he also bestows upon symmetrical specimens of true pohutukawa sometimes found in the forest. These terrestrial specimens of the Rata are usually found in comparatively open places in the forest, while the distorted giants which started in life as epiphytes are usually most abundant, and attain their greatest development in the denser parts, a condition which of itself goes far to account for the comparative rarity of terrestrial specimens. Occasionally dwarf specimens exactly resembling the young cultivated plants, except that they produce flowers, are found on elevated rocky places, but the cause of their stunted maturity is self-evident. It is uncertain if the aerial root of the Rata should be considered simply adventitious or as a special development of the original epiphytic root, although I am inclined to believe the latter. In any case the Rata stands alone amongst New Zealand trees in developing stems of large bulk and affording valuable timber from aerial roots.
[Footnote] * A characteristic specimen, which has been under cultivation for at least fifteen years without producing flowers, may be seen in the grounds of the Honourable James Williamson, Remuera.
The pohutukawa (Metrosideros tomentosa) sometimes produces aerial roots from the main trunk, but these are usually small and appressed. Our President has informed me of a remarkable instance on the west coast of the Great Barrier Island; the plant grows on the summit of a cliff and has given off a root, now become an immense stem, which has travelled down the face of the cliff some sixty or seventy feet to seek its nourishment in the soil at the base. The example is so striking as to have received a special name from the Maoris.
The only tree which the Rata seems powerless to injure is the puriri (Vitex littoralis); a fine example, surrounded by three or four large stems, which it has forced outwards at the base, is to be seen on land belonging to Mr. W. C. Daldy, by the Hotea River, Kaipara; similar instances are rare.
While on this subject I may be allowed to remark that our plant (M. robusta) has been largely used of late years in the place of the pohutukawa for shipbuilding; it is therefore desirable that the attention of shipbuilders and marine insurance companies should be drawn to the fact that for durability it is inferior to the pohutukawa, or even to the rawiri or tea-tree. Should its use be persisted in, considerable discredit will in a few years be brought on our ship yards.* The Rata of the south (M. lucida) is not more durable, and has the additional disadvantage of splitting with the slightest blow. It is remarkable that the pohutukawa and the kauri, the timbers best adapted for shipbuilding in the colony, are practically confined to the province of Auckland, the former only having a single outlying habitat at Waitara in the province of Taranaki.
M. robusta appears to have its centre of distribution in the Kaipara district, where it is abundant, and attains a large size. It occurs from the North Cape to Cook Straits, and has, I believe, been found in the province of Nelson. It is, however, comparatively rare from the Waikato southwards.
I am informed by Sir George Grey that only a single specimen is known on the island of Kawau, although it is abundant on the Great and Little Barriers, Waiheke, and other wooded islands in the Hauraki Gulf.
[Footnote] * Since the above was written I have been informed by a well-known shipbuilder that although M. robusta is not durable when grown on low land or in gullies, yet when grown on hill sides it is equally durable with the pohutukawa.